Increasing demand for mobile and personal communication services has increased pressures for extra capacity in the radio frequency (RF) spectrum while requiring the use of power efficient small sized batteries to power the subscriber units of these communication services. Linear modulation (e.g. II/4 Digital Quadrature Phase Shift Keying (DQPSK) or Quad-16 Quadrature Amplitude Modulation (QAM)) maximizes spectrum capacity but are not necessarily the most power efficient.
Because linear amplifiers can amplify signals with any combination of amplitude and phase modulation, the choice of modulation scheme is not limited by the transmitter and hence can be software selectable. This is advantageous in military applications, and in commercial applications crossing international borders and standards. Other applications for linear amplifications include various digital cellular and Private Mobile Radio (PMR) systems, traditional Amplitude Modulation (AM), Single Side-Band (SSB) systems, instances where transmitter turn-on is to be well controlled such as bandlimited pulse systems found in radar and paging applications, and in Time Division Multiple Access (TDMA) systems.
It has long been known that feedback can linearize non-linear systems. Cartesian Feedback for example, which uses negative feedback to the baseband quadrature modulation provides excellent reduction in intermodulation distortion with low complexity and cost. A typical example of what is achievable with Cartesian feedback is given by M. Johannson and T. Mattsson, "Transmitter Linearization Using Cartesian Feedback for Linear TDMA Modulation", in the proceedings of the 41st IEEE Vehicular Technology Conference, St. Louis, U.S.A. VTC-91 pp. 439-444, May 1991.
Dynamic control of the DC gate bias has also been shown to improve amplifier efficiency by A. A. M. Saleh and D. C. Cox, "Improving the Power-Added Efficiency of FET Amplifiers Operating with Varying Envelope Signals", IEEE Transactions on Microwave Theory and Techniques, Vol. 31, January 1983. U.S. Pat. No. 4,631,491 also demonstrates that feedback can be used to control the collector and base bias in a Bipolar Junction Transistor based RF amplifier to improve the efficiency of the amplifier. More recently, U.S. Pat. No. 5,420,536 demonstrates that dynamic bias modulation may be used by an RF amplifier to maximize spectral control and reduce IF distortion.
However, it is desired to improve amplifier operation and reduce current drain in a low voltage linear amplifier in a TDMA system where dynamic envelope modulation is not used or in any other power amplifier where the bias input is required before the supply.